mov: Fix harmless OOB read.

Convert key_off initialize to use the same sc->keyframe_count as
used elsewhere in the function.
Signed-off-by: Dale Curtis <dalecurtis@chromium.org>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/mov.c

@@ -1842,7 +1842,7 @@ static void mov_build_index(MOVContext *mov, AVStream *st)
         unsigned int stts_sample = 0;
         unsigned int sample_size;
         unsigned int distance = 0;
-        int key_off = sc->keyframes && sc->keyframes[0] == 1;
+        int key_off = sc->keyframe_count && sc->keyframes[0] == 1;
 
         current_dts -= sc->dts_shift;